Authorship and Origin:

It is not known who recorded the words of Haggai, and very little is known about Haggai himself, except that he was a prophet / priest. However each message is very clearly dated and spans from August 520 B.C. until December 520 B.C..

Overview and Significant sections

Haggai's message is to 'rebuild the temple'. He is trying to encourage the people, who have recently returned from captivity under Babylon. Rebuilding the temple is a very important part of establishing the Jewish community in Palestine. This is during the same time as Ezra and Nehemiah. Rebuilding the temple also affirms their faith in God. This book is composed of four short sermons and then reports of the response from the people to these sermons.
For more detailed study:
Read it. (the entire book is available here.)
The Concise Matthew Henry Commentary on this book.
bible.org study of this book.
the World Wide Study Bible has Dictionary, Commentary, Scripture and sermons available on this book.